The present invention relates to profile modification agents for altering the permeability of preselected portions of a subterranean formation. In preferred embodiments, the present invention relates to a new and improved composition and method for profile modification of a subterranean hydrocarbon-containing formation to reduce water:oil ratios and improve petroleum recovery during enhanced oil recovery operations. More particularly, it relates to new and improved aqueous gelable compositions exhibiting high temperature gel stability at temperatures up to about 150.degree. C. and in harsh brines and methods for using same.
The enhanced secondary recovery of oil from oil-bearing or containing subterranean formations by fluid drive processes, wherein a fluid is injected into the formation by one or more injection wells to drive the oil through the formation to one or more production wells is a known process, commonly referred to as enhanced oil recovery. Fluids used in such processes include liquids such as water and various hydrocarbons, and gases such as hydrocarbon gases, carbon dioxide, steam, etc. Many oil reservoirs comprise layers or zones of porous rock which can vary in permeability from zone to zone. In all fluid drive processes, a recognized problem is the predilection of the drive fluid to channel along or through the more permeable zones of the formation. This is commonly referred to as channeling. Another problem is viscous fingering which occurs, for example, by the over-ride of a viscous fluid by a less viscous fluid. The more conductive zones after the oil has been largely displaced therefrom function as "thief zones" which permit the drive fluid to channel directly from injection to production wells. In many instances, such channeling or fingering results in leaving substantial quantities of oil in the less permeable zones of the formation which are bypassed. Such channeling or fingering can occur when the mobility i.e. the quotient of the reservoir's permeability to the drive fluid divided the viscosity of the drive fluid becomes large relative to the mobility of the reservoir oil.
One of the significant problems, therefore, attendant to the production of oil and gas from subterranean hydrocarbon containing formations, is the concomitant production of water. Such produced water can be reservoir water occassioned by coning or a similar phenomena of the aquifier, or it can be injection water from secondary or tertiary recovery treatments being applied to the formation. Whatever the source, there is an upper limit beyond which water production can no longer be tolerated and its further entry into the producing well bore must at least be reduced if further production of hydrocarbon resources at that location is to be continued.
Regardless of whether the undesired water is a natural drive fluid or an artificial drive fluid such as from secondary or tertiary recovery projects, the problem is primarily occassioned by the predilection of the drive fluid to preferentially seek the higher permeability zones and to more or less bypass the lower permeability zones. The mobility of a fluid in a permeable geological formation is the effective permeability of the formation to that fluid divided by the viscosity of the fluid. In the past, a conventional method for reducing the mobility of drive fluids through permeable formations has been to increase the drive fluids viscosity. Such an increase in viscosity is generally accomplished by using viscous solutions of high molecular weight polymers such as polyacrylamides, cellulose ethers, polysaccharides and the like. Such polymeric solutions have been found effective for reducing the water:oil ratio in the total producing well effluent and for increasing the daily production of hydrocarbonaceous fluids.
In actual field practice, however, such mobility altering polymers elute out of producing wells quickly and the water:oil ratios rapidly rise back to an undesirable level necessitating retreatment of the producing interval with the viscous polymer solutions. These viscosity increasing polymers are relatively expensive materials and a one time treatment would be particularly desirable.
More recently, reduction in the permeability of the pre-selected portions of various subterranean oil bearing formations has beem accomplished with gelable solutions of polymeric materials. The formation of gels by the cross-linking of polymers is well known in the art for this purpose. A great deal of literature has been generated concerning the formation of gels in situ in underground formations for the purpose of treating the formations to better produce oil and gas from bore holes drilled into the formations and to decrease undesired water output. It is well recognized that polymer gels and processes incorporating same facilitate the plugging of underground formations in desired areas e.g. by modifying the fluid flow profile, and in particular by decreasing the relative permeability of the most permeable portions of the formations.
Prior art gelling compostions for use in profile modification applications generally comprise water, polymers capable of being cross-linked by polyvalent metal cations and polyvalent metal ion crosslinker. Prior art crosslinkable polymers have included polyacrylamides, carboxymethylcelluloses and polysaccharides, generally of high molecular weight in excess of one million. A commonly used system for generating polyvalent metal ions has been to provide them in the form of chelated metal ion complexes or as part of a redox system. The redox system will generally comprise redox couples wherein oxidizing agent is selected from water soluble compounds of polyvalent metals wherein the metal is present in a valence state which is capable of being reduced to a lower polyvalent state as exemplified by potassium permanganate, sodium permanganate, ammonium chromate, ammonium dichromate, the alkali metal chromates, the alkali metal dichromates and chromium trioxide. Sodium dichromate and potassium dichromate because of low cost and ready availability are the most widely used of the oxidizing agents. The reducing agents in the redox couples have included sulfur containing compounds such as sodium or potassium sulfide, sodium or potassium hydrosulfide, sodium or potassium metabisulfite, sodium or potassium bisulfite, hydrogen sulfide, sodium or potassium thiosulfate, thioacetamide and others, as well as non-sulfur containing compounds, such as hydroquinone, perahydrazinobenzoic acid, hydrazine phosphite, hydrazine dichloride and others. Illustrative prior art profile modification compositions and methods are disclosed in U.S. Pat. Nos. 3,727,687; 3,952,806; 3,964,923; 3,981,363; 4,018,286; 4,039,029; 4,040,484; 4,043,921; 4,110,230; and 4,120,361 to list but a few.
The crosslinkable polymers used in the past have comprised mainly high molecular weight partially hydrolyzed polyacrylamide compounds. A serious shortcoming of the high molecular weight polyacrylamides is that the effective life of the gel as a profile modifier is seriously decreased by the natural temperature of oil-bearing formations having temperature above for example 60.degree. C. and the hydrolysis caused thereby. This temperature effect is further complicated by the significant divalent ion concentrations found in most reservoir fluids, which can cause precipitation of the modifier. Lower molecular weight polyacrylamides which are partially hydrolyzed to about 10 mol percent carboxylate groups have also been used. The higher molecular weight polyacrylamides may be used at lower polymer concentration and hence have been considered more economical. However, the thermal stability of the higher molecular weight polyacrylamide materials is poorer than for the lower molecular weight polyacrylamides and the lower molecular weight materials have exhibited the best stability properties of the materials currently in use. Gelable compositions comprising optimum concentrations of low molecular weight polyacrylamide and cross-linking agent perform satisfactorily up to temperatures of about 90.degree. C. However, at higher reservoir temperatures such as those occurring naturally in a number of locations, such as the North Sea, for example, temperatures at or above 120.degree. C., frequently as high as about 150.degree. C., or often higher, even up to 200.degree. C. are commonly encountered. At these higher temperatures, even the low molecular weight polyacrylamide gelable composition loses all of its strength within a matter of days. Moreover, as has been mentioned, the polyacrylamides and partially hydrolyzed polyacrylamides are susceptible to degradation and precipitation in harsh environment reservoirs containing divalent ions such as Ca.sup.2+, Mg.sup.2+ and Ba.sup.2+. Effective profile modification requires the gels to retain their strength and water diverting characteristics for a time sufficient to accomplish the flood at higher temperatures up to at least about 120.degree. C., preferably up to about 150.degree. C., and especially preferable to about 200.degree. C. in harsh brine environments. At the higher temperatures shorter time periods are required.
N-sulfohydrocarbon-substituted acrylamide monomers and polymers comprising same are known. See, for example, U.S. Pat. No. 3,547,899, which discloses a homopolymer of poly(2-acrylamido-2-methylpropanesulfonic acid). In U.S. Pat. No. 3,679,000, it is disclosed that polymers and copolymers of N-sulfohydrocarbon-substituted acrylamide monomers are useful as mobility control agents, i.e., as viscosifiers in polymer-flooding or fluid drive processes.
In commonly assigned copending application Ser. No. 622,899, filed June 21, 1984 now U.S. Pat. No. 4,573,533, a mobility control reagent comprising an aqueous composition of a polymer consisting of acrylamide units and units of 2-acrylamide-2-methylpropanesulfonic acid or its salts, is disclosed which is resistant to viscosity degradation in the presence of divalent salt containing brines up to or at temperatures of about 90.degree. C. These acrylamide/2-acrylamido-2-methylpropanesulfonic acid copolymers, however, are generally not crosslinkable to form gels, and therefore are not suitable for extended profile modification applications.
Accordingly, it is an object of the present invention to provide a new and improved composition and method for profile modification operations which is effective at elevated temperatures of up to at least about 120.degree. C. preferably up to about 150.degree. C. and especially preferably up to about 200.degree. C., even in harsh environment reservoirs.